ph and buffers Dr. Mamoun Ahram Summer, 2018
Kw Kw is called the ion product for water
What is ph?
Example: Find the K a of a 0.04 M weak acid HA whose [H + ] is 1 x 10-4? HA H + + A - K a = [A - ] [H + ] / [HA] = [H + ] 2 / [HA] = 10-4 x10-4 / 0.04 = 2.5 x 10-7 Example: What is the [H + ] of a 0.05 M Ba(OH) 2? Ba(OH) 2 Ba + 2OH - [OH - ] = 2x 0.05 = 0.10 M = 1 x 10-1 [H + ] = 1x 10-13
Example: The [H + ] of a 0.03 M weak base solution is 1 x 10-10 M. Calculate pkb? B + H 2 O BH + + OH - [OH - ] = 10-4 K b = (10-4 x 10-4 ) / 0.03 = 3.33 x 10-7 M pk b = -log K b = 6.48
Exercises What is the ph of 0.01 M HCl? 0.01 N H 2 SO 3? 0.01 N NaOH? 1 x 10-11 HCl? (this is a tricky one) 0.1 M of acetic acid (CH 3 COOH)? Remember Ka
Determination of ph Acid-base indicator Litmus paper (least accurate) Universal indicator An electronic ph meter (most accurate)
Henderson-Hasselbalch equation pka is the ph where 50% of acid is dissociated into conjugate base.
Maintenance of equilibrium Le Châtelier s principle
A comparison of the change in ph (water vs. acetic acid) 0.010 mol of base are added to 1.0 L of pure water and to 1.0 L of a 0.10 M acetic acid 0.10 M acetate ion buffer, the ph of the water varies between 12 and 2, while the ph of the buffer varies only between 4.85 and 4.68.
What is a buffer? Buffers are solutions that resist changes in ph by changing reaction equilibrium. They are composed of mixtures of a weak acid and a roughly equal concentration of its conjugate base. CH 3 COOH Acid Conjugate base CH 3 COONa (NaCH 3 COO) H 3 PO 4 NaH 2 PO 4 H 2 PO 4 - (or NaH 2 PO 4 ) Na 2 HPO 4 H 2 CO 3 NaHCO 3
Titration curve of buffer What is the midpoint? Equivalence point What is the ratio of the conjugate base:acid at the different points? Buffering capacity
How do we make/choose a buffer? A buffer is made by combining weak acid/base and its salt. Buffering Capacity depends on: Buffer concentration pka of the buffer The desired ph
Exercise A solution of 0.1 M acetic acid and 0.2 M acetate ion. The pka of acetic acid is 4.8. Hence, the ph of the solution is given by Similarly, the pka of an acid can be calculated
Exercise Predict then calculate the ph of a buffer containing 0.1M HF and 0.12M NaF? (Ka = 3.5 x 10-4 ) 0.1M HF and 0.1M NaF, when 0.02M HCl is added to the solution? What is the ph of a lactate buffer that contain 75% lactic acid and 25% lactate? (pka = 3.86) What is the concentration of 5 ml of acetic acid knowing that 44.5 ml of 0.1 N of NaOH are needed to reach the end of the titration of acetic acid? Also, calculate the normality of acetic acid. The number of equivalents of OH- required for complete neutralization is equal to the number of equivalents of hydrogen ion present as H+ and HA.
Titration curve of phosphate buffer Note values
Excercises What is the pka of a dihydrogen phosphae buffer when ph of 7.2 is obtained when 100 ml of 0.1 M NaH 2 PO 3 is mixed with 100 ml of 0.1 M Na 2 HPO 3?
Buffers in human body Carbonic acid-bicarbonate system (blood) Dihydrogen phosphate-monohydrogen phosphate system (intracellular) ATP, glucose-6-phosphate, bisphsphoglycerate (RBC) Proteins (why?) Hemoglobin in blood Other proteins in blood and cells
Bicarbonate buffer Blood (instantaneously) CO 2 + H 2 0 H 2 CO 3 H + + HCO 3 - Lungs (within minutes) Excretion via kidneys (hours to days)
Titration curve of bicarbonate buffer Note pka
Why is this buffer effective? Even though the normal blood ph of 7.4 is outside the optimal buffering range of the bicarbonate buffer, which is 6.1, this buffer pair is important due to two properties: bicarbonate is present in a relatively high concentration in the ECF (24mmol/L) the components of the buffer system are effectively under physiological control: the CO 2 by the lungs, and the bicarbonate by the kidneys It is an open system (not a closed system like in laboratory) An open system is a system that continuously interacts with its environment.
Acidosis and alkalosis Both pathological conditions can be either metabolic or respiratory. Acidosis (ph< 7.35) Metabolic: production of ketone bodies (starvation) Respiratory: pulmonary (asthma; emphysema) Alkalosis (ph > 7.45) Metabolic: administration of salts or acids Respiratory: hyperventilation (anxiety)
Respiratory conditions Respiratory Acidosis H + + HCO 3- H 2 CO 3 CO 2 + H 2 O Respiratory Alkalosis H + + HCO 3- H 2 CO 3 CO 2 + H 2 O
Metabolic conditions Metabolic Acidosis H + + HCO 3- H 2 CO 3 CO 2 + H 2 O Metabolic Alkalosis H + + HCO 3- H 2 CO 3 CO 2 + H 2 O
Causes of respiratory acid-base disorders
Causes of metabolic acid-base disorders
Compensation Compensation: The change in HCO 3- or pco 3 that results from the primary event If underlying problem is metabolic, hyperventilation or hypoventilation can help : respiratory compensation. If problem is respiratory, renal mechanisms can bring about metabolic compensation. May be complete if brought back within normal limits Partial compensation if range is still outside norms.
Acid-Base Disorder Primary Change Compensatory Change Respiratory acidosis pco 2 up HCO 3- up Respiratory alkalosis pco 2 down HCO 3- down Metabolic acidosis HCO 3- down PCO 2 down Metabolic alkalosis HCO 3- up PCO 2 up
FULLY COMPENSATED ph pco 2 HCO - 3 Resp. acidosis Normal But<7.40 Resp. alkalosis Normal but>7.40 Met. Acidosis Normal but<7.40 Met. alkalosis Normal but>7.40
Partially compensated ph pco 2 HCO - 3 Res.Acidosis Res.Alkalosis Met. Acidosis Met.Alkalosis